The invention relates to a method for determining the absorption of a blank for producing an optical element, comprising: radiating a heating light ray through the blank for the purpose of heating the blank, and determining the absorption in the blank by measuring at least one property of a measurement light ray influenced by the heating of the blank. The invention also relates to an apparatus for determining the absorption of a blank for producing an optical element, comprising: a holding device for the blank, at least one heating light source for generating at least one heating light ray for heating the blank, a measurement light source for generating a measurement light ray, and a detector unit for measuring at least one property of the measurement light ray influenced by the heating of the blank. The invention furthermore relates to an associated optical element and to an optical arrangement comprising at least one such optical element.
The radiation used in optical systems for microlithography, in particular in projection exposure apparatuses or projection lenses, which is typically at an operating wavelength in the UV range (less than 250 nm), is absorbed at the surfaces and in the volume of the optical elements arranged in these systems. The material of the optical elements is heated by the absorbed radiation and expands, and the refractive index in the respective material changes in a location-dependent manner, such that a thermal lens effect (“lens heating”) is associated with the change in temperature, and said thermal lens effect can lead to imaging aberrations in the optical systems.
The thermal lens effect can be compensated for actively (with manipulators), but it is more expedient to provide optical elements having reduced absorption, in order to counteract the thermal lens effect by reducing the temperature increase in the optical elements.
In the prior art, the absorption in blanks composed of a lens material transparent to wavelengths in the UV range is typically characterized in transmission. In this case, the internal absorbance coefficient k is determined, which is given by:Kint=−log(Ti)/d, where Ti denotes the internal transmission (i.e. transmission corrected by reflection losses) and d denotes the sample thickness. For the optical materials used nowadays in microlithography, the absorbance coefficient k is generally composed of a stray light component in the range of between approximately 7 and 10×10−4/cm and an absorption component in the range of between 1 and 2×10−4/cm. Given a sample length of typically 1 to 5 cm, an error in the transmission measurement leads to a further uncertainty of approximately 3×10−4/cm in the absorbance. If, by way of example, given a sample thickness d of 3 cm and a measured absorbance coefficient of k=1×10−3/cm, a typical error ΔT in the transmission of 0.2% is assumed (owing to a fluctuation in the surface absorption and owing to the measurement error), the error Δk in the absorbance coefficient k is Δk=2.85×10−4/cm. In addition there is also the uncertainty about the scattering and absorption losses at the two surfaces of the blank. Consequently, it is virtually impossible to reliably measure a volume absorption of less than 2×10−4/cm using transmission measurements.
Therefore, a method for directly determining absorption is required which ideally also allows 100% monitoring of all blanks.
A method for directly determining absorption has been disclosed, inter alia, by the publication by K. Mann, A. Bayer, T. Miege, U. Leinhos and B. Schäfer “A Novel Photo-Thermal Setup for Evaluation of Absorptance Losses and Thermal Wavefront Deformations in DUV Optics”, Proceedings of the 39th Boulder Damage Symposium, Boulder, Colo. (USA), SPIE Vol. 6720, 6720-72 (2007). The photo-thermal measuring device described therein uses cylindrical blanks having a diameter of approximately 25 mm and a length of approximately 40 to 50 mm. A thin heating light ray (having a diameter of approximately 5 mm) from an excimer laser (wavelength of approximately 193 nm) is radiated through said blanks in the longitudinal direction and said blanks are heated in the process. An expanded measurement light ray from a diode laser (wavelength approximately 639 nm), which runs at a small angle (typically 5° to 10°), with respect to the heating light ray, radiates through the volume heated by the excimer laser and the optical material surrounding said volume. A Shack-Hartmann sensor analyses the wavefront of the measurement light ray that has passed through the blank. By comparing the wavefront without and with the action of the heating light ray, it is possible to determine the thermal lens effect with an accuracy of significantly better than 1 nm. Through suitable calibration (computation, electrical heating, gray sample), an absorbed energy can be assigned to the wavefront deformation and the absorptance can thus be determined with knowledge of the incident energy.
What is problematic about the above-described method with virtually collinear heating and measurement light rays is that the wavefront distortion on account of the absorption at the surfaces of the blank is also measured. This requires very careful polishing of the surfaces, which is intended to guarantee a low surface absorption that is reproducible as well as possible.
Furthermore, heretofore it has additionally been necessary to cut separate samples from the blank: since contaminations penetrate into the blank from outside during the manufacturing process, axial and radial edge regions of the blank generally have a higher absorption than the inner region and are therefore cut off before delivery. If the samples were manufactured from edge material, a systematically excessively poor measurement value would be obtained and sampling from the acceptable quality region either would not be nondestructive or would result in a great increase in production costs, since an additional diameter or an additional volume would have to be included in planning, in order to obtain high-quality and thus representative material also outside the geometry actually to be produced.
In principle, the collinear photo-thermal method described above would in principle also be possible on (entire) polished blanks if, for each blank thickness, a calibration and the measurement were carried out shortly after switching on the laser, i.e. if the input heat were still concentrated on the vicinity of the heating light ray. However, owing to the high sensitivity of the method to the surface absorption, a very complex polishing of the entire blank would be necessary, which would be very cost-intensive. Moreover, with progressive improvement in the material properties, i.e. reduction of the absorption in the volume of the blank, the point would rapidly be reached at which the surface absorption would dominate the overall signal during the measurement.
Alternatively, a direct determination of the absorption of a blank can also be carried out with an LID (“laser-induced deflection”) method, as described e.g. in DE 101 399 06 A1. A cube or a parallelepiped having at least four polished surfaces is required for carrying out this method. In this case, the measurement light ray runs transversely, i.e. at an angle of approximately 90°, with respect to the heating light ray and passes the heating light ray outside the light bundle cross section thereof in the volume of the blank. The gradient in the refractive index produced by the temperature increase in the material of the blank produces a deflection of the measurement light ray, which can be detected e.g. by a quadrant diode. This method has the advantage of not being sensitive to surface absorption, but it is very sensitive to the distance between measurement light ray and heating light ray. More recently (cf. C. Mühlig et al., “Characterization of low losses in optical thin films and materials”, Applied Optics, Vol. 47, Issue 13, pp. C135-C142) is has been proposed that the method be carried out with two or with four measurement light rays. Since the latter lie on both sides of the heating light ray, this method is less sensitive with respect to the relative alignment of heating light ray and measurement light rays. Nevertheless, the capture range is only approximately 1 mm, for which reason samples of fixed geometry continue to be relied on.
The article “Absolute measurement of surface and bulk absorption in DUV optics from temperature induced wavefront deformation”, by B. Schäfer et al., Optics Express 2010, Vol. 18, No. 21 has also disclosed an apparatus and a method for quantitatively determining both the surface proportion and the volume proportion of the absorption of a sample. In that case, a heating light ray and a measurement light ray intersect at an angle of 90° within the sample volume, a Shack-Hartmann sensor being used to analyze the wavefront of the measurement light ray that has passed through the blank. In order to carry out an absorption measurement in a spatially resolved fashion, this method also requires highly complex polishing of the surfaces of the entire blank or an additional sample volume.